What is the ruling of Islamic law on purchasing an apartment with an old rental contract from the landlord, and is there any sin in buying it, knowing that the landlord is the one who offered to sell and did not demand the evacuation of the apartment?

If the owner of the apartment offers to sell it willingly, and the tenant pays the market price or the requested price, then the contract is valid and there is no sin. However, if he offers it out of necessity, such as being unable to reclaim it, or if the price paid does not equal its real value and he accepts it because there is no other option, then this sale is void and forbidden, due to the absence of the condition of mutual consent. Allah Almighty says: (O you who have believed, do not consume one another's property unjustly but only [in lawful] business by mutual consent) An-Nisa/29. Thus, a sale made under duress or necessity is not valid.